Since {be'} is a rover, negating whatever precedes it, and since the intended meaning is "someone doesn't find/observe/notice/discover", then obviously the correct choice is {tu'be'lu'} and not {tu'lu'be'}. 

The latter, means "not someone unspecified finds/observes/notices/discovers".

So, why choose {tu'lu'be'} ? Is it because of some canon, which I'm unaware of, or is it because "everyone's doing it" ?

~ nI'ghma