One of the most important tasks during a season that an NFL team is responsible for that largely is conducted behind the scenes is doing the due diligence of monitoring the available talent around the league. This is not just about potential trades and things like that, but also in creating an index of street free agents who would be available to be called upon if needed.
Conducting in-season tryouts with players during the week is common procedure in the NFL, and occurs pretty much on a weekly basis, even if it is not always reported. Often, several players at one time would be hosted together, even working out together.