Hi Laura,

Those are fungus gnats. If the office already had houseplants, it's possible they were already there, especially if those existing plants are larger (based on how many gnats you've found). It hardly matters, as fungus gnats are relatively ubiquitous and not a serious houseplant pest. 

While the larvae occasionally feed on root hairs, the damage is negligible. Fungus gnats are more so an issue because the adults are just annoying. The control methods you have been using are not bad, but there are other methods that are more effective. 

Fungus gnats are a sign of overwatering. The best preventative measure is just to reduce how frequently you irrigate the plants. This will cause the larvae to dry out and die.

Rather than using a tray of water, try using yellow sticky cards. These gnats are attracted to the yellow color. These cards often come with a stake, allowing you to insert them right into the soil of the plants. It can also help you determine which plants are producing the most gnats. 

Rather than using Zevo, consider making a 10% solution of hydrogen peroxide and watering the affected plants with this solution. This will kill the fungus gnat larvae and any fungus in the soil that they are feeding on. 

It's also possible that the plants are potted in a media that holds water too well. If the problem persists despite these solutions, consider repotting the plants into a better draining soil (e.g. something with a higher proportion of perlite, vermiculite, or sand).

Here is a fact sheet on this insect. Let me know if you have further questions.

Fungus gnats - Agricultural Biology