The thought that puts many managers off out-of-hours cleaning is a simple one: someone else in the building, with keys, when you're not there. It's a reasonable worry, and a good cleaning company will have thought about it long before you raise it.
Keys and fobs
Keys should be logged, labelled by a code rather than the address, and held by a small number of named people. Fobs and swipe cards should be individually issued and easy to deactivate. Nobody should be photocopying a key to 'save time'.
If a key is lost, you should hear about it the same day, and the building's locks or fobs handled accordingly. We'd rather tell you straight away than hope it turns up.
Alarms and codes
Alarm codes are best kept individual to each cleaner where the system allows it, so you can see who entered and when. Where a shared code is unavoidable, it should be changed when a team member leaves.
The building's own rules matter too. Many London offices have managed reception and access logs, and a cleaning team should sign in and out just like anyone else.
What you should expect from your cleaner
Vetted, insured staff who work to a written checklist; a named contact you can call; a record of who attended and when; and a clear route for reporting anything unusual, like a door left ajar or a window open.
It's how our team works across our office cleaning contracts, and it's a big part of why we keep the same people in the same building.
You should never have to wonder who has your keys.
The short answer
Log the keys, limit who holds them, use individual alarm codes where possible, and expect proper reporting. If a cleaning company can't explain its key and access process in a few sentences, keep looking.