To think this odd?

[PinkGlitter123]

One shirt hanging up clean and the next time I look there is brown sauce stains on it. (Was hanging up on a door handle ). That could quite easily be explained but then I go into my wardrobe and see my fresh shirt has pen marks over it. Again, no explanation for how it got there.
What would you be thinking?

Poltergeists.

Seriously though, could the shirt have been hung near something that had a pen in the pocket? Or brushed up against something and wasn't noticed before it was hung up?

Twice though?
Starting to suspect someone who come into my house regularly.....

Who comes into your house regularly???

Sabotage? A ghost? A child/sibling/other person borrowing your clothes?

Does anyone else live in the house?

There could be someone living in your loft.

So the pen on the shirt has happened twice? I thought it was once and the other incident was brown sauce which, as you say, has a fairly easy explanation. I once found (apparently inexplicable) ink all over a favourite jumper and it turned out a forgotten biro had leaked in an old handbag, which is why I suggested it may have brushed against something.

First incident brown sauce on one shirt.
Second incident shirt hanging up had pen marks on it, no explanation. Seems weird that it happened twice with no explanation for either.
We have a sitter come in the house as well as immediate family.