Do you have any other reasons to assume it’s dementia.
There are some deficiencies (my grandad had a sodium deficiency) that can cause similar symptoms- sent him temporarily doolally. Also UTIs can do this.

Okay.
For the moment don't try to surreptitiously 'test' him. Just interact as normal.

You might want to make a mental note of how he makes a cup of tea, let him lead wrt breakfast making, observe his decision-making. Make sure he takes his tablets/meds if on any.

A difficult, honest convo to be had with your Mum in the near future about how he is, does she have to do lots for him. Not in front of Dad, no-one likes being talked about.

Nb don't deny his story, it is his reality for the moment, just be reassuring (well they've gone now and taken their silly old smelly fish) and divert (oh LOOK was that a Bullfinch/woodpecker in the Apple tree) and distract (shall we pop to the paper shop to buy a paper)

Can you stay til Mum returns from herweekend away? (Might not be possible ofc)

Ring Doorbell for the front door. Nest camera system covering the back door, you can see comings and goings if set up correctly.

Make sure he drinks plenty, UTI can cause delirium in older people.

Bit of a ramble, stream of consciousness there, sorry.

Was it the smell that has made him think some one has been in? If so there are various things that can send you sense of smell wacky.

The smell of fish can sometimes be an electrical fault.

I had this years ago in a student house. Smell of hot fish on and off for a few days (not all the time), and then the electrics 'went' (bit of a bang, thankfully no real damage).

I don't want to worry you, but that would be my first concern.

Here's some info www.electricaltestingcertification.co.uk/news/why-it-so-dangerous-ignore-fishy-smell

Covid altering his sense on smell and perception?

When my Dad was soon after his diagnosis of dementia he was convinced people were living in his flat. He moved from the master bedroom to the tiniest of rooms as there were 'others' in his room. He used to keep the door closed and knock before he went in. He would be distressed if I just went into that bedroom without knocking (to show him no one was there).

Sometimes even when I was visiting he'd say 'there, they've just gone out and slammed the door, did you seem them? Now do you believe me?'

He'd go out looking for 'them' because they hadn't taken a key. He'd knock on other peoples doors looking for 'them'.

He wasn't safe to be living independently.

How are DM and DD now @penguinparty00? Have you managed to have a talk with DM?

Sorry meant DF not DD.

My mum said people were in her flat when she lived alone, it freaked me out so much. She was eventually diagnosed with mixed dementia Alzheimer's and vascular.

It sounds unlikely someone came in to the house, microwaved some fish and left- in my opinion. Why does he think someone would do that?

My aunt had vascular dementia and used to say there was door in the ceiling, which a man used to come through. She had quite advanced dementia by this stage.
I’d stay with him, interact as normal and make sure he drinks plenty of water.

My MIL has been diagnosed with dementia at 89 years of age, she is now 90 and still struggling. My partner and I have spent the last 18 months taking her for assessments, scans, hospital appointments, talking to the Intensive Dementia Team, doctors and basically anyone who would listen. We have good days and not so good, however, to her, the images are real, so we never disbelieve her, we always make sure that she knows that, but the scenes that she describes can be quite strange. In the beginning she was frightened, but now she isn't so scared. She is on three medications, a patch for anxiety, a tablet for her dementia and a tablet for low mood. Luckily, we only live a 3 minute walk from her bungalow, so we are constantly up there with her. We have put door sensors on her doors so that we are alerted should her front/back door be opened outside the set hours. We noticed that if she hasn't slept properly the night before this triggers an episode whereby she isn't 100%. On the good days, we can still have great conversations, we take her out and about this includes clothes/food/home shopping or even just a day out at the beach, it's just the bad days that are so draining. The other day after a sleepless night because she thought a man was in her bed, she refused to go to bed and instead sort of slept in her armchair, the next morning, she was extremely paranoid about people being in her bungalow, to the point, whereby she was trying to convince us that she was waiting for the people to print off some forms for her that were going to come out of her alarm clock. As you can imagine this is very stressing for her as well as ourselves. My mum on the other hand, reached 90 without a sniff of dementia, her words 'my body has just clapped out', again her words 'she still had all her marbles'. It is very distressing for people who are experiencing dementia as well as family witnessing it with their loved ones. In our experience, we found that distracting her was a great way forward, talking about her past etc always calmed her down. I really hope that all works out for you and your family, you sound like a lovely daughter.

It sounds unlikely someone came in to the house, microwaved some fish and left- in my opinion. Why does he think someone would do that?

The alternative is that he can no longer trust his senses and his brain - that is such a scary prospect that any theory, however fantastic, will seem preferable

It is a classic symptom of dementia, I'm afraid. MIL really is absolutely convinced we spend half our time in her house, hiding stuff, moving stuff, and can't accept how crazy the idea is.

However, it might also be the result of your DF having a UTI which can cause a lot of confusion in the elderly.

You and your mum need to get him to the GP for a urine test, and if you mention your concerns they can do a cognitive test as well.

Fingers crossed for you.