Usually you are not going to find Border Patrol Agents in any establishment unless there has been a complaint by a concernced citizen that there are illegal aliens on the property. As I stated before, an Agent needs suspicion to detain anybody. Giving up ones documents for inspection is equivelent to detention.
The degree of suspicion needed depends on how far away the agent is away from the Border. Suspicion is based on articulable facts. You can't just "know" somebody is an illegal alien. An agent has to be able to explain the cause for dention. Appearance alone does not justify detention. There has to be other indicators that would cause an agent to detain a person.
Suspicion is only required if an agent is going to detain someone. The Agent like anybody else can have a casual conversation with whomever he chooses. The mere presence of a Border Patrol Agent does not equate to detention. If during the course of a casual conversation the agent can develop enough articulable facts for probable cause, the agent can then make an arrest. At the same time the individual approached by the Agent can stop the conversation anytime they choose.
Not exactly --- around here you find border patrol agents having lunch in the same cafes the illegals are in --- I think it's kind of funny to see --- I'm quite sure the agents pretty well know who is likely illegal --- the illegals sitting there will nod their heads almost like greeting and the agents nod their heads back. No one gets their ID's checked --- and I've seen this first hand often. Illegals don't seem at all bothered by their presence and will not hesitate to enter the cafe even though the green vans are plainly in view in the parking lot.