I wouldn't think they'd have the authority to seize the drugs themselves, since the warning sign is from the Port of Portland, the governmental authority that oversees PDX as well as marine ports. My feeling is that they would be obligated to attempt to hold the bag (less likely) or flag the bag and passenger (more likely) for a Port Authority Police officer to handle.

EDIT: According to one story [0], the TSA cooperated as, essentially, an "informant" for the DEA, flagging those with large quantities of cash, for the DEA to then seize or investigate.

EDIT 2: In fact, the TSA policy is to keep dangerous materials off aircraft, but they have no real enforcement authority. Law enforcement gets involved when, for example, narcotics are found. "While TSA works to keep dangerous items off of commercial aircraft, when contraband is found, it must be reported to local law enforcement. Here are a few of the more notable narcotics discoveries:" [1].

0: http://dailysignal.com/2016/01/21/how-the-tsa-and-drug-enfor...

1: http://blog.tsa.gov/2015/01/tsa-2014-year-in-review.html

True, but they wouldn't be able to alert authorities if they weren't actively looking for the items, which gets back to the original issue of speed of inspection.