OP I'll probably be flamed for saying this but you've already done the initial snooping now and as I see it you can't really question him without admitting to this, and once you do he'll just change passwords and remember to log out in future so in your shoes I'd probably check out the situation more at this point by looking at messages etc
I think the fact he's secretive by nature probably causes a lot on insecurity for you but it doesn't necessarily mean it's a dodgy situation.
I couldn't be in a relationship with a naturally secretive person I couldn't handle it, but that's just me I literally have no secrets I tell everyone everything.
The fact he's gotten friendly with her on holiday without mentioning it to you, and added her and she has photos of them both of them on her account that make you uncomfortable makes me think that YANBU to be worried because you are entitled to whatever feeling you damn well want BUT it could all be innocent.
You don't know if his friends were egging him on for the photos or
If she's just a touchy feely person
For those saying you're BU to be upset I don't agree. Your husband knows you, presumably you're the same person he met fell in love with and got married to and you haven't had a sudden personality transplant. So surely in the process of that he has established what you are comfortable with in the relationship and what you don't like so if he genuinely believe that these photos would be enough to hurt your feelings then innocent or not he should have been a bit more respectful of you.
There are boundaries in every relationship some women might be happy for their husband to film themselves dry humping another woman in some relationships a woman draping herself over someone would be too much.,
I wouldnt like to see a photo of a woman draping herself over my dh likewise I'm sure he would not appreciate a picture of me draped over another man. That might not be a big deal to some people but personally I would expect my husband to know where the line is with me as he chose to marry me warts and all with this line in place.