Did my Tenant Abandoned the Property Without Notification?

I have a tenant who paid the entire year's rent. Yep, one year in full. It was one of the tenant's parents who actually paid the money and the tenant gave us a cashiers check. The tenants are in their late 30s.

In any case, we live nearby so we know that they were in the house for at least 6 months but at the end of the 6th month, we noticed that the yard wasn't being kept up. 

I called and text them about another matter, but I received a text stating that they were tending to a family illness.

After they told me about the family illness, I became suspicious. I went by the house to ring the doorbell but the doorbell did not ring. The lights were completely off throughout the house and the back gate was open. 

Although I am still investigating, I believe that they moved out a week or two earlier. I will check with the neighbors to see if they know anything. I haven't entered the house yet, but I will go back to the house this afternoon, knock first, then enter. 

My lease was mirrored after the Texas Real Estate Commission (TREC) lease form, so it is a detailed and thorough lease. 

I was just baffled thinking that is it possible that they actually left the house. They had nothing to lose personally because one of the parent's paid the year's rent. Nevertheless, why wouldn't the tell me that they were leaving? They had already been in the house for a year, so this was a renewal lease.

Has anyone ever had a situation where the rent was paid in full and the tenant left without notice? Again, I am still investigating the matter and will know something by the end of the day but it just reminds me of how important it is to cover all bases on your lease. You never know what people will do. Your comments are welcome.