The only dumb question is the one you don’t ask – agreed!
Should the Silvia have any `built up` steam pressure with the machine being left on and idle for some time – and why is it not consistent?
If the Silvia has been left on and idle for several hours (or longer) – and with the heating lamp off - if I open the steam knob I get a large release of pressure through the steam want. i.e. 20 - 30 seconds of wet steam
Other times I get nothing – not even a whisper.
Is this normal?
Yes, it is normal.
These machines like all domestics are controlled by thermostats rather than by pressurestats. Effectively in terms of pressure, you get whatever you get when the machine is left on for extended periods, and in practice yes, you get a "slight" build up of pressure which is that which we all purge by temp surfiing the group before placing the group handle to make coffee.
Just because the machine is thermostatically controlled and can be left on all day, doesnt mean that leaving the machine on all day is what the designers had in mind.
Basically, its a fact of life and nothing to be concerned about.