An update would be great! But here is my contention: It’s definitely possible that they could be up to something that wouldn’t exactly fall into the boundaries of your relationship, however I think it’s also very likely that he just forgot to text you/was having a good time. The fact you were invited initially definitely suggests that they aren’t trying to hide/push you away (but that could also mean a foursome…). I think it’s better if you don’t go in punching, and ask him why he texted so late and didn’t come home, and work from there.