I believe used fecal bags from the LM were normally supposed to be transferred back to the CSM and returned with the crew to Earth for analysis. 

In the [Apollo 15 flight journal][1], in the annotation after 174:14:00, we have this:

> [As Apollo 15 disappears behind the Moon, Al switches on the Gamma-ray Spectrometer, X-ray Spectrometer and the Alpha Particle Spectrometer. The crew transfer all required items from the LM aided by a list in the Flight Plan that includes notes on where each item should be stored. The list includes film magazines, bagged rock and soil samples, core tubes including the very long deep core, food, used urine and fecal bags, and one of the OPS packages from the surface.]

That annotation was added after the fact, but in the [Apollo 16 flight journal][2], at 191:19:09, while preparing to close out the LM for jettison, CAPCOM Haise gives the crew a checklist of items to transfer to the CSM: 

> 191 19 09 Haise: Okay. Another little block you might write out to the left there, label it "Transfer Items." And, maybe you've already done some of these, but this will take care of some of the ones we missed having you do yesterday. And they're the PTK(?) [PPK], the Flight Kit, the purse with the unused food, and the used fecal-urine bags, and lastly the DSEA.

This doesn't speak directly to the earlier flights, but given that the used bags on the CSM were retained for analysis from day one, I think this would have been the procedure for all the moon landings. It also doesn't even prove fecal bags were used on 16; Haise almost certainly meant "if any were used, transfer them, and please don't tell me about it". 

I can't swear Aldrin didn't toss a used diaper onto the moon, but I am strongly leaning towards "Myth: Busted" here. 

Since the earlier flights involved shorter lunar stays, [it's less likely that fecal bags were used at all][3]; having to do one's business in the cramped LM cabin next to your crewmate would not have been pleasant. [Lomotil][4] and willpower can get you through two days pretty easily.

Table 2 in [SP-368 Biomedical Results of Apollo][5] appears to enumerate all the collected fecal samples from the Apollo missions, but unfortunately doesn't give timestamps or origin locations for them; it is also thus inconclusive. 


  [1]: https://history.nasa.gov/afj/ap15fj/18tunnel_leak_lm_jett.html
  [2]: https://history.nasa.gov/afj/ap16fj/24_Day9_Pt1.html
  [3]: https://space.stackexchange.com/questions/32208/did-astronauts-on-the-moon-poop-in-front-of-each-other?noredirect=1&lq=1
  [4]: https://en.wikipedia.org/wiki/Diphenoxylate
  [5]: https://history.nasa.gov/SP-368/s6ch2.htm