cause for concern or just jealousy?

I would definitely be suspicisious. That's my nature, generally, but that sounds fishy to me. I would ask him about it. Not in an accusing way, granted. You don't want to go into what could be a casual, harmless conversation looking for a fight. Just be like 'hey, I've noticed that you took down our picture. Are we okay?' If he tries to shrug it off and gives you an excuse that you know isn't right, then bring in the archives being shut off or his party invite. I know in my relationships I don't like starting fights if I can avoid them, and it could be just him getting sick of conversations being logged, wanting to update his picture, and him being lonely while you're at work... I mean, it doesn't say 'hey ladies, who wants to come chill' it says 'who wants to come party'. That's slightly less suspicious, I think.

Hi friend, first of all how confident are both of you about the relationship? Generally, I have seen couples deeply in love for a long time put pictures of two of them like a mutual agreement on their profiles. If he had asked you to party at house and you refused, then may be thats his fantasy wish and now looking elsewhere while keeping you as the pure gf. you can put some general talk about other people having parties at home etc etc. that you overheard and see his positive or negative comments. The cat might come out of the bag!!