Using IC (internal combustion) forklifts indoors – whether they are propane, diesel or gas powered – can lead to dangerous exposure to carbon monoxide.
However cold the warehouse may be make sure it is ventilated – i.e. keep that big door open – WorkSafeBC has guidelines on this, including service and emissions testing.
Modern forklifts have better emissions controls but there are a lot of old stinky machines out there. If the air smells bad and looks bad it probably is bad.