Yes Charlie and I do think its a territorial thing with the going in cupboards etc - they are treating the home of the step-parent and their parent as if its still theirs when they just lived with their parent - in these instances.
Its almost like a 'I am going to carry on exactly as I would if this was just my dads home - like the last one was.'
My DHs older son - whom I actually get on with really well and is independant and an all round good egg (unlike his younger brother who still lives with us) - even he used to do this when we first moved in - when he visited he had to knock obviously as I had put my foot down about keys only being given to 'residents' - but even then once he was in he would switch back to 'I'm at my dads' mentality. First time he visited he went in the fridge and took a can of coke and started to drink it and just carried on talking. I didnt say anything but remember thinking am gonna have to nip this mentality in the bud - quite frankly if you have followed any of my threads or postings you will know I have enough to put up with from his resident brother!
The reasons I didnt say anything were as follows - firstly I had done well to get my now DH to agree that his non resident son couldnt have a key - we'd had many heated debates about it but it would have been a deal breaker for me - so I felt to then whinge about a can of coke on only his first visit would have been provocative in the extreme to my DH.
Secondly I do not get on with his other son who lives with us and we had several teething problems with the three of us and the new dynamic of his son living in 'our house' as opposed to me 'the girlfriend' living in his sons dads house - IYSWIM. Had I pushed it on the visiting son fridge raiding issue then I feared DH would have thought - hang on a minute she just doesnt like either of my sons. This would have invalidated any complaints I made to him about his younger son - and there have been many. I had to pick my battles as they say.
Last but not least I really do get on very well with his older son and I felt it was just a case of him not switching dynamic as mentioned above rather than aggresively refusing to acknowledge me IYSWIM - so as he already knew it was down to me that he wouldnt get a key (he accepted it without too much fuss tbf to him) then I just thought well why make him feel uncomfortable about going in the poxy fridge and also he only calls for a few hours every week or two in any event.
As it turned out when he had gone my (now) DH turned to me and laughed and said 'did you see our xyz go in the fridge cheeky bugger and get a can of coke?!' Next time he visited he actually pointed out to him that he should ask - its me and Sudas house this you know dont be so rude - but in a lighthearted way - and ever since then his oldest does always ask - 'Can I get a drink for the kids etc etc'.
Final word on this key issue - one of the reasons I so dug my heels in was the affect on privacy. I said to (now)DH - so because one of your sons lives with us there will only be the odd occasion when I can run downstairs in my undies or in a towel and make a brew etc - i.e. when his nibs is out. Also thats the only time me and you can have some private time without having to retreat to our bedroom. But by your other son having a key that would mean even those odd precious occasions would be spoiled by the possible interruption of the other sons key going in the door at any moment. Thats what it is more than anything - the possibility of an interruption. No I'm sorry but I only think residents of a house should have a key - except maybe give one for emergencies but only on strict conditions - they dont just use it to walk in whenever they want.