could be a birthdate, birth year, address (on gas station signs, license plates), their name or relationship to us (bro, mom)
some people believe seeing a cardinal, butterfly, dragonfly is a sign from a loved one.
a reference to a connection you had with them, finding or running into an item that the two of you talked about or connected over or had an inside joke about
hearing a song the two of you sang together, danced together, or a song they liked.
not to mention when a light flashes or goes on or off, or you smell something related to them.
sometimes talking to them out loud might get their attention, telling them to give you a sign.
keep looking, keep paying attention. 
btw--i wondered about paul's du user name and googled towelie. i see it was a character on south park (that i didn't watch very often) with the catchphrase "don't forget to bring a towel" (looked on youtube and saw towelie for the first time. this made me laugh:
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