What do you think it means when a girl from my husband's job texts him "*" and nothing else?

Twice today, once the other day. I don't know if I'm being a crazy jealous female or if it seems like a test text to see if he's available. I really have no idea and I'm trying not to go off the deep end about it and I'd appreciate some perspective.

I admittedly have a problem with jealousy but he also has a problem with being dishonest.

Also, he asked me to check his phone for him on his day off because he's STILL sleeping and he gets work calls so when this girl started blowing up his phone with *'s, I was checking to see if it was a work text.