This thread may belong in the legal section rather than general handgun...
I'm not a lawyer, and this shouldn't be construed as informed advice. This is some guy anonymously posting on an Internet forum...
In Texas:
If the state mandated 30.06 sign is not posted in clear view at the entrances of the building, legally you can enter while carrying a legal concealed firearm. This assumes that it is one of the few types of property where carry is prevented by law, such as schools, hospitals, federal buildings, rodeo grounds (really) etc.
However, in the event of an arrest/prosecution, a police officer or judge is likely to take a dim view of your actions if you had to walk past one or more well marked entrances so that you could sneak in an unmarked back door.
Also remember that in Texas, the property owner can compel you to leave, with police assistance if necessary, for carrying a concealed firearm against their wishes, even where legal. They may not be able to bring weapons charges, but they can make you leave the premises.
All of this applies if you get caught. Be discreet. Also, I make every effort to avoid businesses that have any type of no carry policy, whether said policy is legal or not. They obviously don't want my business so I choose to spend my money elsewhere.
Edit:
Stony beat me to the punch, using half the words to do it.