Every bicycle I have ever owned from childhood until now I have had to pump the tires up to riding pressure before each ride or at least every week.

I pump up and check my tires every time I ride out of habit. I also spin the wheels to see how the bearings feel, to see how true the wheels are, and to see if the brakes are grabbing.

It takes less than 10 mins, and I know the wheels are in good shape.

Two issues.

• Typical loss of air through a tube. With a standard black butyl tube one can lose a little pressure, but it can take months for a tire to bleed down to zero. Even so, it usually has a bit of a spongy feel to the tire.
• Puncture. Can go flat from a matter of seconds to a few days. If left long enough, all air is generally lost, and there is no spungyness left with the tire/tube

All of your descriptions of your actions make it sound like you're treating it like the first condition with slow air loss.

On the other hand, your description of what is happening (losing all the air during a ride, or overnight) puts it soundly in the second condition with a puncture.

You can just swap tubes, and keep going. But I like to determine the cause of every flat.

I mark or remember where the valve is on the tire (or some people orient it with tire markings such as pressure ratings or logos). Inflate the tube to moderate pressure 2x or 3x normal size, and look/feel/listen for a puncture. If it is a slow leak, then immersing a small portion of the tube and looking for bubbles in a bucket of water can help.

Once you found the leak, compare it to the tire and rim to determine what actually caused it.

Flats can have one of a number of causes.

• Snake bite. Find mirror image two holes close to each other. Typically a pinch flat from running over some road debris with too low of pressure or too much weight. Maybe being lazy and letting tire pressure to bleed down over a month, then getting a pinch flat.
• Rim side flat. A tube if not twisted will have part facing the rim. A flat here could mean a burr from manufacturing, or bad rim tape.
• Radial Tire Wires. Look for a very thin wire sticking through the tire near where you got the flat. They can be extremely small, but sharp as needles.
• Glass can chew its way through the tire rubber and into the tube. Periodically I'll take a few minutes and pick all the glass out of my tires. But, also pay attention to flats. I am also careful with what I run over.
• Crooked Valve. Especially with Schrader tubes, the rim can cut through the valve.
• Goat heads or thorns. Problematic in some areas.
• Tire installation errors. Either snagging the tube with a tool like a screwdriver, or getting a section of the tube pinched under the bead when installing.
• Leaky valve. Rare, but also the easiest to check. Just spit on the valve and watch for bubbles.
• Blowout. Loud pop. Usually raged edge of large hole in tube. Often either a sidewall failure, loose bead, or a tube that was impinged during installation. Maybe a large hole in the tire that wasn't booted.
• Unknown. A small percent simply won't have a discernible cause. It should be the minority. If most flats fall into this category, then you aren't looking.

Finding the cause of a puncture can be a bit of a pain, but if you don't figure it out, it will keep coming back, whether it is something in the tire (radial tire wires), or something you've done (pinch flats).

Any shop that puts all the flats in the "unknown" category is doing a poor job and a disservice to you. Even if they're not patching tubes, they should figure out the causes of the flats.

I glanced through all the answers and maybe I missed it, but there is another alternative: extra thick tubes, usually known as thorn proof tubes.

I had one tire where the tire lost air over a very long period of time. I took the tube out, inflated it and dunked one section at a time under water whilst looking for bubles that'd indicate a leak. It was not until I pumped that tube up to a VERY large diameter that a bubble appeared. It took many seconds for each bubble to reappear once i wiped a bubble off. It was an almost microscopic puncture cause be an extremely fine piece of wire in the tire.

Cheers