There are other 'trails' visible from aircraft - what are they?

Wake trails: As an aircraft passes through a lower troposphere having a high relative humidity, (usually during landing or take-off phases - and for military aircraft, during 'high-G' manoeuvres), very short, non-persistent 'trails' can sometimes be seen coming from the wing tips, or white 'lift-generated sheets' streaming off from the trailing edges of the main wing, control surfaces etc. Both features are due to short-term local reduction of pressure, leading to condensation, though the precise mechanism in each case is different.

(a): "lift-generated sheets": as an aircraft moves forward, air accumulates (pressure builds), at leading edges, with a compensating depletion of air (fall of pressure) across the top of the wing (generating lift) and along trailing surfaces. The reduction of temperature in the near-saturated environment, consequent upon the slight lowering of pressure, can be enough to cool the air to it's dew point, and thin sheets of water droplets are observed.

(b): "wing-tip trails": the flow of air around the wing-tips undergoes marked distortion which manifests itself as a tight-vortical (or 'twisting') motion of the airflow; the vortices are formed by, and will lead to, a local increase and decrease of pressure - in the latter case, if the atmosphere is humid enough, then white trails can be observed. In both cases, the sheets/trails (of minute water droplets) will evaporate quickly again due to mixing with the non-saturated environment in the wake of the aircraft.

Dissipation trails (DISTRAILS): In contrast to the formation of CONTRAILS (see FAQ here ), aircraft on passage at high levels can cause the dissipation of pre-existing cirriform cloud, due to the local increase in temperature consequent upon the ejection of hot exhaust gases from the aircraft engine. The passage of the aircraft will be marked by a clear lane in the cloud. However, it will be obvious from the description (above) relating to condensation trails, that the heat outflow must markedly outweigh the injection of water vapour from the spent fuel, and the phenomenon is rare. The effect may also be caused by turbulent mixing with dry air just above the cloud layer, caused by the aircraft motion, and this mechanism can lead to temporary clear lanes in other cloud forms, e.g. thin stratocumulus or altocumulus. However, beware of a similar phenomenon, whereby the shadow of a 'normal' condensation trail is cast on thin cirriform cloud below - leading to a visibly dark band in the cloud. This is not a dissipation trail.

Incidentally, whilst on the subject of 'trails', if you are looking at visible satellite imagery over the region of a slow-moving anticyclone, and notice lots of thin, white lines criss-crossing the region, which don't appear on the corresponding InfraRed image, these are ships' trails, caused by exhaust particles from the vessel acting as condensation nuclei, and 'seeding' the humid, near sea surface environment, and betraying the presence of the ship by a thin band of water droplets which are not dispersed due to the very light winds and minimal mixing in the anticyclone.