Why are helicopters looking for radiological anomalies near Trump’s inauguration?

Yes, helicopters are used a few days before an event to get a map of the background radiation of a site. The helicopters are flown over the event the day before or the day of the event again to see if there are any anomalies.

Radiation is very difficult to detect from the air due to the Inverse Square Law, but a helicopter might be able to detect a very large radioactive source such as a Dirty Bomb. Nuclear weapons are very well shielded to protect the operators, so it would still be very difficult to detect nuclear weapons from the air.