Is a prior/recent entry into Mexico a red flag for U.S. immigration officials?
I have recently finished watching all of the episodes of the TV show Border Security: Canada's Front Line. I know you asked about the States, but in this regard, I would say that the immigration practises are not too dissimilar.
I have observed two things:
- Mexico: considered to be a drug source country. There are a good number of cases shown whereby visitors to Mexico and other South American nations bring back banned substances to their country of origin. This happens knowingly as well as under duress. Substances tend to be concealed within the body at times and this can only be brought out of the traveller under extensive questioning/checks.
- India: I've heard immigration officers state that India is also a drug source country especially when packages are shipped from India. Another flag: it is not uncommon to find Indian visitors bring in undeclared food/fruits/meat products.
Is so, why and is it fair?
I think this to be a matter of opinion. Every country has their own rules on who they want to screen based on circumstantial clues. There are going to be people who think it makes a place safer and there are going to be people who think it is a nuisance.
I have my opinion too: immigration officers have a job to do regardless of a travellers country of origin, gender, race, etc. If you feel like you have been treated unfairly, there are channels through with grievances can be reported.
I do suspect they treat a Mexican entry as a risk, but I'm looking for concrete citations that they do.
That is not in the public domain. A lot of the time, immigration officers tend to question persons they feel could be concealing something.