In the past few months, I've seen many clients receive random text messages and WhatsApp messages from unknown numbers. Often it's just the word "hey" or sometimes they say something vague about having met at a party or plans to go golfing. If they really are someone you know who wants to say something let them do that, don't ask for more information.
From what I've heard, these are often "romance scams", where they will reveal supposedly personal information in the hope that you will reciprocate. It may escalate to nude photos. They may then use this information to blackmail you.
Even if you only want to check whether you know the sender, the safer precaution is not to reply to an unexpected message. Engaging could give a scammer an opportunity to draw you into a conversation or try another tactic. These could involve impersonating a relative in trouble, impersonating local law enforcement (they know your area code so they could make an educated guess as to your local police department), or impersonating a bill collector, bank, or delivery service.
On the flip side of this—if you are texting someone you haven't texted in a long time or someone you recently added to your phone, it might be useful to include some context rather than just a "Hey" or they might just ignore you.