not unusual and is a pre-cursor event to a lightning strike. You're lucky to be alive! Next time, drop fishing pole, and hunch down, not sit down. Wrap your arms around your knees and make yourself into a ball as low to the ground as possible. Also back away from the water line, but not under/near trees or poles(high objects). I know that the geographic circumstances don't always allow all the above to be done.
As a means of providing some level of warning, carry an AM transistor radio. At the sign of inclement weather, you want to tune between AM radio stations anywhere on the dial, and listen for the lightning approaching. You will be able to hear cloud to cloud, and cloud to ground strikes.
The park events are most likely caused by what clothing and shoes you were wearing, and the ambient humidity. Did the shock occur when the detector coil came close to the steel support poles of the playground equipment?
Keep in mind the levels of ESD(electrostatic discharge) you described could possibly damage the detector. I would suggest touching the coil to the ground every couple swings to dissipate the charge before it has a chance to build to the level you described.
HH
BarnacleBill