Can an opinion from a police officer be admissible on the basis that the question is foundational?

Of course, the opinion may be admissible on another issue, such as to explain the officer's subsequent conduct. (See People v. Brown (2001) 96 Cal.App.4th Supp. 1, 34.) And, in fact, when defense counsel raised the point later in the trial, the court explained (as it apparently had done, off the record, at an earlier point) that its ruling was based on its view that the question was foundational. The problem with that explanation is that there does not appear to have been any question of impropriety in conducting a further investigation, so the foundation also was irrelevant. Beyond that, a limiting instruction should have been requested and given if the evidence was being received only to explain the officer's subsequent conduct. But nothing was said at the time about the evidence being admitted for a limited purpose; it came in "at large."