(From the Annie Proulx story) "Got to tell you, friend, maybe somebody seen us that summer. I was back there the next June, thinkin about goin back -- I didn't, lit out for Texas instead -- and Joe Aguirre's in the office and he says to me, he says, 'You boys found a way to make the time pass up there, didn't you,' and I give him a look but when I went out I seen he had a big-ass pair a binoculars hangin off his rearview." He neglected to add that the foreman had leaned back in his squeaky wooden tilt chair, said, Twist, you guys wasn't gettin paid to leave the dogs baby-sit the sheep while you stemmed the rose, and declined to rehire him.
Jack implies to Ennis in the motel room in 1967 that while he went to Aguirre's office in Signal, he just went there to see if Aguirre was hiring; but, he was not quite sure he wanted to work for Aguirre again. . . and because Aguirre mentioned their finding a way to spend the time of there, Jack might have known he knew they were not doing their jobs. Notice that Jack did not tell Ennis that Aguire refused to hire him again or even why.
While the movie has Jack seeing Aguirre with the binoculars in his hand up on Brokeback, Jack knows absolutely nothing about the binoculars until 1964 and he does not even see them until after Aguirre talks to him and he leaves the trailer. Annie Proulx wrote that they did not even know that anyone was watching them goofing off (doing more than just having sex), aka "stemming the rose," neglecting their assigned duties.
I get the impression from the text after Aguirre watched them the first time, that 10 minute period of watching, he observed what they did after that, too. I can sort of read between the lines that after Ennis opened up to Jack after they switched places and believe that they started goofing off for quite a while before they had sex the first time which was after they had been up on the mountain for quite a while. It was well into summer before the first night in the tent session. Say . . . maybe 6 to 8 weeks after they started work.
While the book does not say so, I think that Aguirre was actually a foreman over more than just one sheep herd crew up in the mountains around Signal.