senior dog sneezed blood

hello, i have a senior male chihuahua (he is 13) and today he sneezed a little bit of blood. i took him to the emergency vet straight away of course. the vet said they cannot locate what’s wrong and he seems okay, and to just keep an eye on him, maybe use a humidifier. when i look it up online there is a lot of scary things that come up, especially for senior dogs (i also know looking up symptoms online should be taken with a grain of salt.) the vet said he is okay, but i just am very, very nervous and obviously do not want anything to happen to him, he’s my childhood dog. this vet also has been my dog’s vet since he was a puppy, and he is amazing, i believe he is very good at what he does. should i get a second opinion, or go with what the vet says and just keep an eye out?