UA agent here.

Not condoning the agents demeanor or attitude, just addressing your bag issue. On the concourse, typically we do not investigate issues regarding missing luggage. We can absolutely check to see where the last scan was, the bag routing and whether it's where it should be. But we do not coordinate with the ramp or other departments if a bag is determined to be missing or perceived to be in the wrong place. Generally, these issues are dealt with at your final destination baggage claim. What the agent should have done was checked the tag, if they determined there was an issue then advise you to address it with baggage service at your final destination.

At hubs and larger stations bags are typically not staged at the gate, they are kept in holding areas and on UAX flights it is not uncommon for bag loading to begin after boarding starts. At smaller stations and outstations where the ground crews work both the gates and ramp area interchangeably, they may be better equipped to locate missing bags but this is not practical at the hubs and the larger line stations and where I work we do not investigate bag related issues on the concourse.