Last night I left a half-eaten plate of food on my bedroom floor. This morning I get up for work, and notice it covered in ants…and I notice another horde of ants under my desk, which is about 10 feet away. Since I was running late, I decided I’d deal with them later. So I just got home from work, and there’s still an army munching on my leftovers, and JUST AS MANY OF THEM hanging around under my desk, where there is neither food or a visible escape path. What is the deal with this second army? Surely they’re up to something…or at least they WERE before they were vacuumed up…
Maybe an **invisible **escape path? Small gap in the floor or wall or around an electrical outlet?
Do you eat at your desk?
When I have doggy-type visitors, even though I’ve just vacuumed carefully, they always seem to find interesting smells and crumbs under my desk, where I usually take my meals. Perhaps the ants have found the remains of a snack or droplets of a beverage that are unapparent to mere human beings. 
I do occasionally eat at my desk. Guess I better get out the mop.
Sugar ants likely. Now that they have found a food source and communicated back to the hive, you will probably not get rid of them without chemical measures.