I had a beer at 7pm on Sat. the 28th of Oct. Took an in home ETG test on Nov 2 and tested
Positive. 120 hour later. Is this possible?

Yes, it is possible.

Obviously it's possible, false positives happen all the time. Did you retest?

Yes it’s possible, and it might not be a false positive.

J9 is correct as there are medical reasons you may have to pay attention to if you test positive after a few days of drinking ONE beer. RETEST, and maybe see a doctor for further tests if the result is the same.

Retesting is the only way to decide if the first test was indeed a false positive, or a mistake by you, or the testing product you used. Many courts are bound by law to confirm a positive drug test result, and medical professionals will order confirmation tests also.

I advise you do the same.