The Broncos bring guys in every week at various positions and work them out, not much ever comes of it. Like last week for instance, even with all their struggles in the return game, they took a look at Jeremy Bloom. He was a great returner at C.U. and a local kid to boot, so it seems he would've been a nice fit as a KR/4th WR or something, but they didn't sign him.
Yup. This is just the modus operandi for the Broncos.
Denver is a team that, like the Redskins, tries to primarily build with veteran talent. Unlike the Redskins, though, Denver doesn't blow its wad over the big-name free agents. Instead, they rely a lot on 2nd-tier veteran depth mixed in with some stars. For instance, their defense features a lot of retreads on the D-line, mixed in with young studs (or prospective studs) like Dumervil and Moss. It features retreads at LB, mixed in with the young stud D.J. Williams. It features retreads in the secondary, mixed in with Champ Bailey. Denver has always been a big recycling bin for veteran players. Some of them (like Bertrand Berry, Nick Ferguson, and Kelly Herndon) get a second chance at life and become amazing success stories. Some of them (Luther Elliss, Courtney Brown, Ellis Johnson) are essentially non-factors and quickly find their way out of the league.
Anyway, because this is Denver's roster-building strategy, their scouting department is configured to optimize it. They keep dossiers on every single rookie they scout, including the ones that they never draft, and when those rookies hit free agency, even if their career was disappointing to that point, Denver pulls out the dossiers and brings in the ones they were originally high on (which is why Denver leads the league in number of former first-rounders on the roster). In order to keep those dossiers up to date, Denver is bringing in new players every single week, giving them a workout, and updating their notes. Generally, Denver focuses on a position a week. This week, it looks like that position was running back.
I wouldn't get too worried, personally. Like I said, this is just business as usual for Denver. It never makes any headlines except for the one time that the players they bring in happen to be runningbacks. Regardless, it doesn't mean anything any more than it meant anything when they auditioned new LBs and returners earlier this season.