On Oct 6, 2018, at 10:08 AM, Simon Brodbeck via INDOLOGY <indology@list.indology.info> wrote:
 
Can anyone tell me the source of the detail that while Krishna was enjoying himself with the gopis, Balarama was watching from behind a rock?

Dear Simon,

The detail is more likely to have come from the oral tradition(s) of kīrtanakāras or hari-kathākāras than from any written source. These performers are known to answer queries from the audience occasionally in a light-hearted way. The Hindu audience is ordinarily quite comfortable with the liberties they take in the middle of a serious, uplifting discourse. 

In the detail to which you refer — Balarāma watching intimacies without himself being seen — may also  indicate a  liberal attitude toward voyeurism. 

It is also possible that the kīrtana or hari-kathā performer did not wish to come across as a deficient knower of the tradition. In other words, if the traditional sources did not raise a question that an inquisitive member of the audience thought of, he had the presence of mind to improvise an answer — that he was not one who did not know answers to all questions.

A few statements about Balarāma in a similar vein can be found on pp. 275-277, 319 (last 5 lines) of Lee Siegel's _Laughing Matters_ Chicago: The University of Chicago Press, 1987. 

a.a.