ventilation, illumination type and intensity, human presence.
Gravity
Not gravity unless they're studying mice in space with some expectation that the subjects are unaffected by the vast different in gravitational pull.
"Apparent gravity on the earth's surface varies by around 0.7%, from 9.7639 m/s2 on the Nevado Huascarán mountain in Peru to 9.8337 m/s2 at the surface of the Arctic Ocean"
That's what I was going for, gravity has probably almost never been controlled for.
Might as well add magnetic field to the list. In the odd circumstance it's possible there could be a large amount of power / electricity in wiring nearby, creating a magnetic field that's different than other test groups, etc..