Temporarily deviating from IFR

You're flying happily over the desert in your small plane, in bright, clear
weather, on your IFR flight plan. You notice a plume of smoke a short
distance away at your nine o'clock, and looking more carefully, you see what
looks like a stranded car with some people standing around it. You'd like to
investigate just to see if it might be someone who needs help.

Given that you're on an IFR flight plan and under ATC control, how do you
handle this? I presume you can't just tell ATC that you're going to swing
south and drop to 2000 feet for a while to check something out. Do you ask
ATC to vector you towards the stranded car? Do you cancel IFR to go take a
look? Is there a way of suspending IFR temporarily so that you can leave your
planned route and altitude and check out the source of the smoke? What's the
procedure?

On a related note (whence the crosspost to r.a.p.), has this ever happened to
you? That is, have you ever seen something that looked like a possible case
of people in distress on the ground and decided to go over and take a look
and/or report it to ATC?

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