I have/had a nice friendship group of four couples. We did a lot of things together, from a bike ride or sporting events to meals out, concerts and holidays. Both as couples and separately in male/female groups.
DH died, they were all "heartbroken", sent flowers, anything we can do etc.
Funeral was arranged for three weeks after his death. I took them up on the offer and asked a couple of people, who bake, to make cakes for afterwards. All seemed fine, they'd be pleased to do it.
Then they all disappeared and looking back, some usually very active SM was silent. In the week before the funeral I heard nothing from anyone. I was getting worried about the catering so messaged to ask who was making what? I got replies saying sorry they're going to have to be bought, no time. Which is ok, but if I'd wanted bought I'd have bought them or if I'd known earlier I'd have baked myself. I genuinely thought these people wanted an opportunity to do "something".
On the day they turned up with those bar cakes from Tesco, definitely not what I'd have bought, but did the job. (And one of them took the uncut cakes home with them!) It didn't matter, there was loads of cake other family and friends had made, which is why the lesser ones didn't get cut, but it felt like they couldn't do this one small thing that they'd agreed to do for me or for DH.
Anyway, the cake's not the issue 
I've since learned that they were all on holiday that week. It's not the fact that they went without me, I wouldn't have gone that week and I'm not sure I'd want to go on a couples holiday anyway but they kept it such a big secret. I've seen them lots of times since, asked what they've been up to etc (individually and as a group) and no one's mentioned they went on holiday! I'm not sure if I'm most upset that they felt they couldn't tell me or they think I'm so stupid I wouldn't work it out! Also, a good friend in the week before her husband's funeral, wouldn't you keep in contact, even if you were away? It's true I didn't contact them either.
Anyway. I've been cordial. They've been "useful" in that they give me people to go places with (one of the hardest things about losing DH is that you've lost that easy company for an ad hoc outing) but I don't feel close to them or trust them at all any more.
They don't know I know and I feel like they're laughing at how they got away with it. I don't know how to broach it with them without getting very upset, which won't achieve anything, they clearly don't care. Although I suppose you could say the secrecy shows they do feel bad about it.
An over reaction that I should snap out of?