I was hand carrying my daughter's oil painting on thick paper -- the entire canvas is of white cap ocean waves. I put it down to get down my bag and forgot it on or under the seat. I am devasted -- it is precious to me. She signed it "Areta '17" in the lower right corner. I was on a flight on April 15, 2018 from SJC to PHX no 6479 that stopped in SAN.
Sorry you left your daughters painting on the plane
have you filed a lost item report with southwest? If not I would do that online and you could also call the airport where you landed in case a flight attendant saw it and turned it in to the gate agent.