Cat clearly not happy about my holiday!

[Nothungrycat]

I go away a few times a year, and either have a catsitter or a neighbour popping in to feed my cats. Neither are that fond of other people, so whoever feeds them doesn't usually play with them (in fact one of them is rarely seen by visitors other than a furry arse exiting through the catflap..) I came back this time to discover that one of them has been pooing in the room I use as an office - probably about 5 times over the 2 weeks I was away. No sign (or rather smell) of any wee, and the poo was pretty normal - so unlikely to be illness related. I'd obviously like to work out why so i can stop it happening again. Has anyone got any experience of similar, and/or any advice?

So many questions...

How many cats?
How many litter trays?
Do the cats ordinarily get along with one another?
Where are the litter trays kept?
Did your cat sitter empty said trays often enough?
Are neighbouring cats able to come into your home via the cat flap?
If you don't have litter trays, do the cats go outside for the necessary?
Was the weather particularly bad in your area while you were away, so the cat decided it would rather not go outside to do its business?
Do you have Feliway plug-ins?

There are two cats, and they get on ok - a little bit of hissing at each other but there's enough space that they can avoid each other. They're outdoor cats (especially one of them - its her safe space) so no litter trays at the moment - there has been one in the past and it was only used by one cat - I did bring it back a few months ago when that cat wasn't very well, but when I took it away there wasn't a problem. We do have a bit of an intruder problem, but its mainly to eat their food, and, yes, it was very windy for a couple of days... So, I suppose there could be a scenario where one of them got anxious because of an intruder or the weather - and I wasn't around to provide reassurance/chase off the intruder...

I'm a firm believer that cats should always have access to a clean, indoor space to go to the toilet, even if it goes unused for months on end.

You have two cats who don't really get along all that well and who aren't keen on strangers being left home alone with a stranger - they need as much security as possible and that includes a safe indoor toilet.