I don't know if you've seen elderly people take pills, but they often have poor eyesight coupled with poor motor skills so losing a pill or two is not surprising. We put my elderly dad's pills in a daily pill holder (one for morning pills, one for evening pills) because he had trouble counting them out himself. Though if a pill is taken on demand, like a painkiller, leaving them in the bottle may be more sensible.
As for how it ended up on the passenger's side, I would hope that if grandma is on narcotic painkillers that she's taking them while she is a passenger, not driving.
As for wether or not someone can be convicted for even one small pill, yes, it is possible... for example:
In NJ: Possessing any amount of a schedule I, II, III, or IV CDS incurs a fine of up to $35,000, at least three (and up to five) years in prison, or both. Possessing any amount of a Schedule V CDS incurs a fine of up to $15,000, up to 18 months in prison, or both. Using or being under the influence of any CDS not for the purpose of treating a sickness or injury (as legally prescribed by a licensed physician) incurs a fine of up to $500.
http://www.criminaldefenselawyer.com/crime-penalties/federal...