I recently watched a YouTube video from someone who'd tracked a nasty intermittent parasitic draw [0]. The problem only happened after turning the ignition on & back off (so the standard test of disconnecting the battery & reconnecting via the ammeter wouldn't show it), and the draw cycled between 3.6A and 0.3A.
Also, after turning the ignition on & back off there were a lot of (normal) transient draws (from things like the dome light that don't turn off immediately). Even with the problem circuit disconnected, it drew around 6A (!) immediately after the ignition was switched off, dropped to 0.4A after about a minute, and sat at that level for another 9 minutes before dropping again to 0.06A. That means if he hadn't waited ~10 minutes per test, he'd have been chasing draws that were actually normal.
Combine an intermittent fault with intermittent normal behavior, and you've got a troubleshooting nightmare.
Also, after turning the ignition on & back off there were a lot of (normal) transient draws (from things like the dome light that don't turn off immediately). Even with the problem circuit disconnected, it drew around 6A (!) immediately after the ignition was switched off, dropped to 0.4A after about a minute, and sat at that level for another 9 minutes before dropping again to 0.06A. That means if he hadn't waited ~10 minutes per test, he'd have been chasing draws that were actually normal.
Combine an intermittent fault with intermittent normal behavior, and you've got a troubleshooting nightmare.
[0] https://www.youtube.com/watch?v=rVScppKsfHs