Left or right depends on whether it's the trailing cab of the lead unit or the lead cab of the trailing unit. It sounds like the one on the secondman's side. But regardless, that door should really be shut when running in multiple, but the hole in the floor that the catch sits in fills up with detritus so it doesn't always engage properly (even in the cab that's in service when the door should be locked open - then it keeps sliding open when you brake and shut when you take power - if I can't get it to lock I just stick my bag in the way as it gets very annoying very quickly!). There are a few switches on the non-drivers side (I'm being deliberately coy about what they do) but they require an EP key to operate. And if anyone did tinker with them, the driver would receive an immediate alarm via MITRAC.
But if you do find one of those doors unlocked, please let a member of the train crew know. Thanks.