I was about an hour later home last night for various reasons, but strangely no comment from the cats - usually at least Girlcat will come running to me crying for attention, while I get very reproachful looks from Tobias.
Found out this morning they were just biding their time to evince their disapproval. Vomit consisting of undigested biscuits, a bit of gritty (biscuit) pasty stuff, and the largest hairball I've ever seen, spread right across the top of the stairs. At least a) it wasn't spread in front of the loo door (possibly because Tobias likes to lie there guarding it for some reason) or b) left on the first step down from the top, where I probably wouldn't have seen it until too late.
However it was all made up for in my eyes when Tobias actually ran to me when I got downstairs for his morning bum and shoulder scratch.