Smoke detector - which electric breaker?

Lots of them have a battery backup in them in the event that power goes out. Guessing you checked that and you mean it's still drawing from the line?

Also, if this is in a commercial area a lot of times those detectors will be tied into the building alarm box that usually has it's own breaker inside.

Can you see where the feed into the SD is? Does it seem to be coming from a nearby light or plug outlet?

You could carefully twist undone the pigtails within the SD box [you said you wanted to move it anyway] and locate the hot feed with a pigtail light fixture or an electric tester. With one person on that end and the other at the panel, I guess it's within shouting distance, you can go thru all the appropriate panel breakers.

If you are in the UK you need an electrician to do it, if you are letting the property and do not undertake this work in accordance with the Regulatory Reform (Fire Safety) Order 2005 you can be held liable under common law (i.e. jail time). Its part of the building regs, get professional advice from the RICS. UK only. NB posted this under the wrong heading earlier.