Dimonnd I would't do anything at this point unless it starts giving you problems during your everyday drives.
A sitting car can weaken the battery, especially if the battery is old.
Next time you let the car sit for a few days, do the following as an experiment.
1) If possible, use a multi-meter and get the volts off the battery before attempting to start the car. Hopefully it's 12 volts or above.
2) When you go to start the car after it's been sitting, try cycling the key a 3-4 times before attempting to start it. Just to make sure the fuel system is primed.
*In case you or someone else reading does not know what cycling the key means, see below *
Cycling the key means turning the key from the off position, to the "run" position and back to the off position. The "run" position is the last key potion right before you start the car.
So....in your case,
a) insert the key
b) next, click the key forward 1 position, this should turn on your radio and unlock your steering wheel.
c) Then click the key forward again 1 more position. This should activate all the lights on your instrument cluster (also known as the Run position).
d) Then, click the key all the way backwards to the off or beginning position.
e) repeat steps b, c, and d two or three more times.
Your goal is not to start the car during key cycling. It's to make sure the fuel pump has fully primed the system.
Give us an update if you let the car sit again, or if you start to have the problem more frequently.