The watching eyes effect has been hotly debated in recent years. Interestingly, field studies often reveal strong effects of eye images on various forms of prosociality. Here, we explored the watching eyes effect on prosociality - in terms of lost letter return rates. We conducted three field experiments, in which we dropped a total of 540 letters in 13 different neighborhoods in the Netherlands (six of high SES and seven of low SES). The envelopes had printed stylized eyes, flowers (or no logo, for Experiments 1 and 2). Combined analysis of all three experiments revealed no significant effect of eye images on lost letter return rates. There was also no interaction between eyes and SES but also no main effect of SES levels on lost letter return rates. Overall, these null results suggest that minimal cues to being watched may not be that powerful in promoting forms of prosociality that entail a certain cost and do not have obvious benefits for the individual, such as return of lost letters.