For how long Robin's been tele-working it has to be something pretty serious. The only mention of anyone on the show of busting her balls about not being in was Richard during the flooding and he got slammed hard and no one backed him up so I think everyone on the show knows what's up and it's serious enough to not make fun or really even mention it.
It is odd that they wouldn't just come out and say whats up though. The show is about truth and the lives of everyone on the show. I guess they got their reasons but still al ittle odd to me