"Detectable" is a funny thing. If a 1 watt laser (for example) could not be detected with 1 hour of staring, it might be detectable with 10 hours. We can already detect signals carried by just a few 100 photons per second (Lunar Laser Ranging). At the typical distance to Mars, a laser there would have to send about 2.6 x 10^9 photons per second. That's only about 30 nanowatts.
On the other hand, at 20 nanowatts, the information rate would be less than 1 bit per second. The noise from the vibration of the laser (from winds and Mars quakes) would blur the signal so much, the laser would not be detectable any more. Better go up to 20 microwatts, and get about 30 bits per second. Still very slow.
When we bounced radar signals from Earth to Mars and back, we had to stare at Mars for about 30 minutes to detect the return signal.
They were going to send an orbiter with laser communication (Mars Telecommunications Orbiter) capability but it got canceled. It would have used a 5 watt laser. That was to send a gigabit per second of data. You could detect a much smaller one.