|I have a Citrus sinensis 'Washington' that is growing indoors in an 18|
|Stippling on the leaves and a sticky substance certainly sounds like the result of insect feeding. Aphids are fairly easy to spot; mites, whiteflies, thrips and scale insects are a little more difficult to detect. Since you've thoroughly inspected the plant and can find no signs of insect pests, the critters may be flying away or may be so tiny you can't see them with the naked eye. Whiteflies scatter when you touch the leaves or branches so look for tiny white specks; spider mites look like little specks of pepper and they generally leave tell-tale webbing where the leaves meet the stems. Inspect the undersides of the leaves with a magnifying glass to see if you can find these critters.
Without knowing which pest you're dealing with, it's difficult to suggest a control. I'd give the plant a periodic shower (tepid water, making sure to gently rub the undersides of the leaves to dislodge any insect pests) and continue watching for insect activity.
Best wishes with your citrus!