Is there any case law or updated opinion on where a sign needs to be posted? The statute states "a sign or signs posted on the property or at the entrance to the building, reasonably likely to come to the attention of intruders, indicating that entry is forbidden", however, I am unable to locate anything that indicates an exact location on the property, such as a field. If a sign is posted on the driveway leading to a field, and someone enters the property on the side (not protected by a fence), where no signs are posted, do they have an argument against being notified about their entry?

There is no exact location requirement. "Reasonably likely" is the key here:

Morgan v. City of DeSoto, Tex., 900 F. 2d 811 (5th Cir. 1990)(quoting Bain v. State, 677 S.W.2d 51, 58 (Tex.Crim.App.1984) overruled on other grounds). (See also, Holcomb v. McCraw, 262 F. Supp. 3d 437 (W.D. Texas June 27, 2017)):

quote:

whether the signs posted on the property indicated that entry was forbidden and were reasonably likely to come to the attention of intruders. It was not necessary that the sign be seen by the arrestee or that it be visible to them from any particular place. It was only required that there be a reasonable likelihood that a sign would "come to [their] attention."

Bain v. State, 677 S.W.2d 51, 58 (Tex.Crim.App.1984) (overruled on other grounds):

quote:

Appellant's contention that he was not guilty of criminal trespass, even if true, is irrelevant since the presence of the no trespassing sign gave Coulson probable cause to arrest. His contention that the sign was not visible from the train is likewise irrelevant since the statute only requires a reasonable likelihood that the sign be visible to intruders.

Fundamentally, whether posted signs are "reasonably likely" to be seen by an intruder is going to be a fact-question, so there is definitely an argument to be made; whether it is a good one or not is the real question (and a very fact-specific one).

Some cases worth looking at to help gauge the quality of your specific facts:

• Castro v. State, No. 08-16-00354-CR (Tex.App.-El Paso, Jul. 31, 2018)(not designed for publication)
  

  quote:

  Defendant claimed he did not have proper notice because he did not use the path that contained the no trespassing sign, but instead hiked through a different route and thus, did not see any "no trespassing" signs. Officers testified it was possible to hike through different areas where one would not see signage, but signage would've been clear once he arrived at his destination. Court found that Defendant's close proximity to the location, evidence that several no trespassing signs were posted close to the location-which were likely visible from where Defendant was—and the fact that Defendant's car was parked near one of the signs on a nearby road all support that Appellant had proper notice.

  
  
• State v. Beckman, No. 04-12-00405-CR (Tex.App.-San Antonio, Jun. 5, 2013)(mem. op.)(not designed for publication)
  

  quote:

  Record indicated two "No trespassing signs" on the property, but no evidence they were posted in a manner "reasonably likely to come to the attention of intruders." Investigator testifying at suppression hearing was not aware of the signs until a few days before the hearing

  
  
• Rue v. State, 958 S.W.2d 915, 918 (Tex.App.-Houston [14th Dist.] 1997, no pet.)
  

  quote:

  defendant had notice and was aware that he was trespassing because of the presence of a no trespassing sign on apartment unit nearest the street

  
  
• Newman v. State, No. 05-93-00583-CR, (Tex.App.-Dallas Mar. 30, 1994, no pet.)(not designed for publication)
  

  quote:

  five posted "no trespassing" signs and appellant's conduct could persuade a rational trier of fact that Defendant acted knowingly and intentionally

  
  

End of the day, it is likely that a lot will depend on what kind of reasonable inferences can be drawn from surrounding facts (Defendant's behavior; prior knowledge; etc.).