Al asked:
last month my sister and i (25 and 23 respectively) got stopped in customs after landing back in england from spain (we’re british citizens). we had queued for ages as it was very busy, showed our passports etc then as we were about to go through to collect luggage, a man in a suit stopped us (looked like a detective or something). he was pleasant enough and asked where we’d been, where we worked in england etc, looked at our passports. all a bit suspicious in my opinion! everything was fine but i didn’t notice him stop anyone else, why did this happen? we did look tired admittedly!

Did the man identify himself to you in anyway? It does seem a bit odd, but if he did it within eyesight of Passport Control, its prolly just a random check…….
Hard to say, without being there myself.
Customs officials mostly employ behavioral profiling in deciding whom to further question. The criteria is based on several things and is difficult to say what the official saw that made him want to investigate further.
However it appears you passed the test, as you were not further questioned.
It’s not a big deal at all. Sometimes they do randomly stop people for no reason at all, just to keep everyone guessing.
As this was after immigrations where you have showed your British passports, and customs. I would guess that he was a customs officer. You do collect your baggage before customs, then you pass customs. So something is wrong in your question where you claim that you collect your baggage after customs.
However, Spain is a major drug distribution area because of Canary Islands and other free trade zones. I guess you were targeted for a random questioning where the officer evaluated if you fit the profile for a drug smuggler. This can be done by just asking some questions and he probably just observed your reaction. He then decided that you didn’t fit the profile and backed off.