It's a problem as long as there's a reasonable chance it could have been switched.
I know of a yeshiva that ordered a dozen schwarmas, which were delivered by a non-Jewish worker from the schwarma place a few blocks away. The delivery wasn't wrapped or taped or anything, and the schwarmas inside were generic enough that they could have come from anywhere, and thus they were declared non-kosher.
Often, when picking up an order from a kosher restaurant today, it will come wrapped or taped or stapled or something, and often there is a paper slip outside the bag listing its contents, as well as the time and date. I've heard that some combination of those factors makes it tamper-resistant enough to pass muster with most kosher organizations, who will have formal standards about this kind of thing. (But as always, ask your rabbi.)
As for your cleaning lady, if she could have reasonably been rummaging through your fridge, taken out a kosher steak, and replaced it with a non-kosher steak (how are the steaks packaged/labeled?) without anyone noticing, then you'd have a problem. Usually that's not a realistic concern.
Back in the day when you had to soak and salt your own kosher meat, the question often came up of the non-Jewish domestic help who claims to have soaked/salted it, can I believe them? This means that s/he was left alone with the meat for a while, yet the meat remains kosher as it's a situation where it couldn't have plausibly been switched.
See Rabbi Heinemann's comments here