R134a is heavier than air, so a leak can leave pockets of it sitting in the footwells, deep in the vent systems etc. When you turn on the blower, it gets moved about. It dissipates pretty quickly and once it's gone, the leak is possibly too slow to leave a detectable smell. Until it sits again....

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)