
sich einschleichen
to sneak in
To go to a place secretly, quietly, and unnoticed.
Der Fuchs hat sich nachts in den Hühnerstall eingeschlichen.
The fox sneaked into the henhouse at night.
to sneak in
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to sneak in
To go to a place secretly, quietly, and unnoticed.
Der Fuchs hat sich nachts in den Hühnerstall eingeschlichen.
The fox sneaked into the henhouse at night.

to creep in
Used figuratively for something negative, such as an error or a bad habit, that develops slowly and unnoticed or becomes part of something.
In den langen Text haben sich leider einige Tippfehler eingeschlichen.
Unfortunately, a few typos crept into the long text.

to ingratiate oneself
To gain a person's trust or affection in a subtle or flattering way, often with dishonest intentions.
Er versuchte, sich mit Komplimenten bei der neuen Vorgesetzten einzuschleichen.
He tried to ingratiate himself with the new supervisor by giving compliments.
'Einschleichen' (to sneak in) is a separable verb. The prefix 'ein-' separates in a main clause and goes at the end. The verb is irregular: einschleichen, schlich ein, eingeschlichen.
Ein Fehler schlich sich in die Berechnung ein.
An error crept into the calculation.
The verb consists of 'ein-' (into/in) and 'schleichen' (to move quietly and slowly). The literal meaning 'to move secretly into something' is the basis for all figurative meanings.
Die Katze schlich sich leise ins Haus ein.
The cat sneaked quietly into the house.
This verb is almost always used reflexively with 'sich'. This applies to the literal meaning (to move secretly into something) and to all figurative meanings (errors, habits).
In meinen Text hat sich ein Tippfehler eingeschlichen.
A typo crept into my text.
The preposition indicates the meaning. 'Sich in etwas (accusative) einschleichen' means to get into something unnoticed. 'Sich bei jemandem (dative) einschleichen' means to sneak one's way into someone's trust.
Ein Virus hat sich in das Computersystem eingeschlichen.
A virus has crept into the computer system.
In the figurative meaning, 'sich einschleichen' (to creep in) is very often used for negative, unnoticed developments. Typical subjects are: an error, a misunderstanding, an inaccuracy, or a bad habit.
Mit der Zeit haben sich einige schlechte Gewohnheiten in seinen Alltag eingeschlichen.
Over time, some bad habits have crept into his everyday life.
to sneak
Both verbs describe moving quietly and unnoticed toward or through a place. Schleichen mainly emphasizes the slow, cautious manner of moving. Sich einschleichen additionally emphasizes secretly getting into a place. Use schleichen for the manner of movement and sich einschleichen for getting in unnoticed.

Der Mann schlich sich unbemerkt in das leere Gebäude ein.
The man secretly sneaked into the empty building without being noticed.
Der Mann schlich leise durch den dunklen Flur.
The man quietly sneaked through the dark hallway.