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It's a closed system otherwise you'll not have bite when you grab the lever.
No, you misunderstand - the technical term used to describe the workings of 99% of the hydraulic brakes fitted to push bikes is ‘open’.
Google it.
When you pull the leaver at a point near the beginning of the stroke the master cyclinder closes off the reservoir, allowing pressure, hense my comment about rubber banding the leavers down if you are concerned about air in the reservoir migrating to the callipers when the bike is transported or stored.
A very good bleed will result in none or very little air in the reservoir. However as the pads wear fluid migrates from the reservoir to the lines and calliper. That fluid is replaced by something as it’s absence means a vacuum . Usually it’s compensated for by the reservoir diaphragm collapsing but air can and does enter the reservoir to partially or completely fill the space that was taken by the fluid.
TL:DR
Perfect bleed by a shop pro? Sure put your calipers above your leavers.
Imperfect or gone a very long time between bleeds? Close the levers before you hang your bike up.
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Howard
It's a closed system otherwise you'll not have bite when you grab the lever.
And a small bubble should not cause the pull to be spongy. Small is however subjective. And the only reason why it's squishy is that the bubbles got into the caliper instead of being at the reservoir in the caliper. Few pumps and gentle raising the wheel then dropping it should move the bubble back up.
So yeah if you've bled it well, there shouldn't be no issue if you flip your bike upside down or hang the bike up by the rear wheel cos no bubbles...