I had it happen before. I wasn't fortunate enough to have misplaed it. I came out to go to work and the door to my Jeep Cherokee was open as was my center console. I had a little CZ70 .32 truck gun in there and it was gone. I reported it stolen.

Give it a day and see if you remember what you did after you settle down a little. If you are sure thats where it was, call the PD and get a report. If they find it or it is recoverd you will get it back. You have the serial number?