A seconds argument is specified in the OP’s second example, but it’s in the wrong place in the argument list. I believe he’s referring to this statement in the WakeOnMove.ino example in the AssetTracker library:
System.sleep(WKP, RISING, TIME_PUBLISH_BATTERY_SEC, SLEEP_NETWORK_STANDBY);
TIME_PUBLISH_BATTERY_SEC is defined as:
const unsigned long TIME_PUBLISH_BATTERY_SEC = 4 * 60 * 60; // every 4 hours, send a battery update
I don’t see any version of the sleep command in the docs, that supports this order of arguments. Is this an undocumented feature, or an error in the library example?