You (or the AI) can do it even if they don't have Stealth.

I've been reporting this bug since the early days of FE.  Stardock has no plans to change it.  Whenever you load from a saved game, all of your units will move if they had been previously been given a move assignment before you saved.  That will results in all kinds of chaos such as you and enemy units standing on the same tile.  I think you've also pointed out a related issue that if you get attacked while you have a previous move assignment, then you will end up standing on the lair tile without looting it.  Since I've been telling them to fix it since early FE, I wouldn't get my hopes up.

I had something similar. A monster walked into a tile next to my city at the end of my turn. In the new turn a unit was built in my city and placed in this tile. The Spearmen and the monster ended up in the same tile without a fight. I've also seen Monsters and AI units walk onto the same tile. I thinks this is a threading issue. The actions of the Player, Monsters and AI are computed in parallel on multi core systems as I understand. So they check where to move at the same time and they see the same tile as free and they decide to move onto this tile at the same time and so they end up both on a formerly free tile. This threading approach seem to create a lot of synchronization problems in several areas. For example sometimes a Champion auto moves onto a quest location at the end of the turn which causes a popup, I close the popup and the map switches to a city I need to level. I level the city and then I get some other popup for example from some treasure I looted. After that it goes back to the quest location and goes on with the quest. I think it is a real mess. This should be addressed. It is confusing and makes a bad impression.