Non-invasive pressure measurement:

FBG (Fiber Bragg Gratings) Strain Gauge - operates by measuring deformations of the tank and interpreting those deformations as changes in pressure (e.g. the tank deflating means the pressure is decreasing). The gauge sits on the outside of the tank and stretches with the tank. Inside the gauge is an optical fiber, which shifts the wavelength of light passing through it as the fiber stretches or contracts.

Pros: 

• Immune to electrical interference because you're measuring light and not circuitry (ie this is pre-radiation hardened)
• is extremely light
• the gauge itself requires no power
• multiple sensors can be multiplexed (used in tandem) with no additional power cost for the interrogators
• takes up little volume

Cons:

• they require an interrogator (the optical equivalent of a ADC) to produce the light and process the incoming signals into something that can be recorded and processed by the system
  • this itself can be mitigated with a low-power interrogator (e.g. Micronor, PhotonFirst) so long as you aren't worried about sampling continuously and are certain that the tank won't deform outside of a certain range
    • if you go with a low-power interrogator you actually make the payload last longer in space since simpler electronics aren't as sensitive to the space environment

In principle, you only need to collect data every few seconds or even minutes because the deformations we aim to track happen pretty slowly. Therefore, this provides a good method of tracking pressure losses caused by outgassing.

Non-invasive leak detection:

Ultrasonic contact sensors - operates by measuring the vibrations of the tank. When a leak (ie puncture) forms on the tank, the jet of air escaping vibrates the tank and you can detect these vibrations. One of these sensors sits on the outside of the tank and passively detects the vibrations

Pros:

• Passive instrument, meaning that the only power it needs is in the ADC
• takes up little volume
• temperature resilient
• already proven as useful in space environment (they're one of the tools used on the ISS to detect leaks)

Cons:

• on my momma I can't think of any